1. Field of the Invention
The invention relates generally to optical transmission systems and more particularly to an optical transmission system having transmit and receive filter passband wavelength offsets optimized for best BER performance.
2. Description of the Background Art
Optical communication systems play an important part in modern society's exchange of information. In present metro and long haul optical fiber systems, 2.5 and 10 Gbit/s bit rates with non-return to zero (NRZ), return to zero (RZ) and chirped RZ (CRZ) formats are widely used. Recently, duobinary modulation is being developed to provide narrower optical spectrum and improved tolerance to dispersion. The narrower optical spectrum of duobinary modulation enables systems to have a greater number of channels at closer spacings and carry more data than the above mentioned formats.
Fiber transmission systems, especially wavelength division multiplexing (WDM) and dense wavelength division multiplexing (DWDM) systems use optical filters at the transmit and receive terminals, as well as other locations in the link where traffic is added and dropped, for multiplexing and de-multiplexing. The option for closer spacing of the channels for duobinary modulation requires more precise passband properties for the filters.
Bandwidth limiting occurs when the passbands of the filters are equal to or less than about three times the bandwidth of the signal spectrum. When the bandwidths of the filters are properly chosen this bandwidth limiting can improve the bit error rate (BER) and the related Q performance of a duobinary signal.
Where cost is of no importance, the best BER performance is obtained by having the center of the filter passbands and the center of the signal spectrum at exactly the same wavelength. However practical considerations such as temperature, aging and the difficult of achieving tight tolerances in manufacturing result in wavelength offsets between the centers of the filter passbands and the center of the signal spectrum. These offsets can induce a significant penalty in BER.
Unfortunately, the BER penalty caused by the offsets is accentuated in systems when the filter passbands are narrow enough to take advantage of the BER improvement due to bandwidth limiting. This problem is particularly acute for systems using duobinary modulation because of the capability for narrower channel spacings.
There is a need for a transmission system and method for minimizing the BER penalty caused by filter offsets without the higher costs for tight tolerances for manufacturing, temperature response and aging of the filters and the light source while retaining the BER improvement of optical filter bandwidth limiting.